More than $2 billion is spent annually treating and managing approximately 5.3 million dry metric tons of biosolids from publicly owned wastewater treatment plants in the United States (WEF/U.S. EPA Biosolids Fact Sheet Project, Biosolids: A Short Explanation and Discussion, 2000). Land application of biosolids, often to food crops, has gained popularity and widespread approval from the scientific and environmental communities and has increased over the years. National biosolids generation rates are estimated to reach 47 dry pounds per American yearly. In 1972, 20 percent of all U.S. biosolids were land applied, while 40 percent went to landfills. By 1995, 36 percent of all yearly biosolids were land applied, while 38 percent went to landfills. The remainder of the material was incinerated or surface disposed. Disposing of biosolids by shipment to landfills is considered a beneficial use only when such disposal includes methane gas recovery for fuel. Methane operations are relatively rare, however, which establishes land application for soil conditioning and fertilizer as the primary beneficial use of biosolids.
The basic aim of waste material treatment processes is to economically and efficiently reduce and stabilize waste sludge solids. In addition, the sludge treatment system should also produce an end product that is fully suitable for final disposal without further physical or chemical treatment. In conventional practice, final sludge disposal is commonly carried out by incineration, land filling or land spreading. In many instances, land disposal is employed and is particularly attractive due to minimal long-term environmental effects and is highly advantageous in reconditioning of the soil. However, the use of land spreading as a final sludge disposal method may require a well stabilized and pasteurized end product; the concentration of pathogenic organisms in the sludge must be sufficiently low to avoid becoming a health hazard. Also, the sludge should be adequately stabilized to prevent further degradation in the environment and the attraction of vectors.
Traditionally, three distinct processes have been widely utilized for treating wastewater sludges: oxidation ponds, anaerobic digestion and aerobic digestion. Oxidation ponds are generally employed in the form of comparatively shallow excavated earthen basins that extend over a large area of land and retain wastewater prior to its final disposal. Such ponds permit the biological oxidation of organic material by natural or artificially accelerated transfer of oxygen to the pond water from the ambient air. The use of oxidation ponds, however, has limited utility, since their operation requires sizable land areas. Moreover, no significant reduction of the level of pathogens in the sludge and only limited reduction in the quantity of the waste sludge is accomplished by this elementary treatment and disposal method.
The process of anaerobic digestion has generally been the most extensively used wastewater sludge digestion process for stabilizing concentrated organic solids. In common practice, the combined excess waste sludge is accumulated in large covered digesters where the sludge is mixed and naturally fermented anaerobically for about 30 days. The major reasons for the widespread commercial use of anaerobic sludge digestion are that this method can be used to: (1) stabilize large volumes of dilute organic slurries; (2) produce significant biological solids (biomass) reduction and stabilization; (3) produces a final sludge, wherein water is easily removed, for ultimate disposal; (4) produce methane gas; and (5) produce a pasteurized sludge under the right conditions. Anaerobic digestion is characteristically carried out in large scale tanks that are more or less thoroughly mixed, either by mechanical means or by the recirculation of compressed digester gas. Mixing rapidly increases the rate of the sludge stabilization reactions by creating a large zone of active decomposition. In the past, the use of anaerobic digesters has been limited due to high energy demands, which translates into high cost for the end user.
Biosolids also are known to contain pathogens that include viruses, bacteria, and various parasitic organisms. There are generally two classes of biosolids recognized in the United States Environmental Protection Agency's (EPA) regulations: Class B pathogen reduction standards, as set forth in 40 C.F.R. §503, which require a fecal coliform level of less than two million most-probable-number (MPN) per gram of total solids, and Class A pathogen standards set forth in 40 C.F.R. §503. EPA's Class A biosolids requirements are satisfied when fecal coliform densities are less than 1,000 MPN per gram total solids; or when Salmonella densities are less than 3 MPN per four grams total solids. Additionally, enteric virus must be less than 1 plaque-forming unit per four grams of total solids, and helminth ova is less than one viable helminth ova per four grams of total solids.
The majority of applications of anaerobic digestion to wastewater sludges have been in the mesophilic temperature range, from 35° C. to 40° C. (95° F. to 104° F.). Anaerobic sludge digestion in the thermophilic temperature range from 45° C. to 65° C. (113° F. to 149° F.) has been practiced to only a limited extent. The limited use of anaerobic digestion at temperatures above the mesophilic range may due to higher energy requirements to obtain the higher thermophilic temperature.
Meeting Class A standards will significantly increase the opportunity for biosolids recycling, however, known processes that achieve Class A pathogen densities in biosolids are generally costly, and in most instances, cost prohibitive. Therefore, a need still exists for methods and processes that can produce Class A biosolids in an efficient and effective manner.